This invention relates generally to wind turbines, and more particularly to methods and apparatus for efficiently reducing load and providing yaw alignment in wind turbines.
Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted on a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 30 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox or directly coupled to the rotor. The gearbox, when present, steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid.
Horizontal wind shears and yaw misalignment, together with natural turbulence, are important causes of asymmetric loads on a wind turbine rotor. These asymmetric loads together with those from vertical wind shears contribute to extreme loads and the number of fatigue cycles accumulated by a wind turbine system. Asymmetric load control could be used to reduce these effects. However, during grid loss conditions, there is no power available for the load control systems. It has been difficult to increase rotor diameters to improve wind energy capture because the larger rotors would have to be designed to accommodate the extreme loads and fatigue cycles during times when grid power is lost and no load mitigating active control can be applied.